Welcome to the Apple Community.
You can't remove accounts from devices you don't have access to, all you can do is change your password.
Messages are nothing to do with the iCloud account, you can remove the ID used for messages at settings > messages.
But I don't understand, like I half have access to the user of the device but they're a million miles away! I tried to get them to delete it but they said icloud isnt showing up for them to delete it! (confusing I know)
But the Ipad only has a credit sim card in it but can send and receive messages to and from my contact,s mean while ive never used that device I've only restored it... I think it is all to do with icloud. So i dont understand what your trying to say...
They're clearly using my icloud? otherwise how else would they be able to do that?
They may well be correct that they aren't using your iCloud account, as I said messages is not iCloud, iCloud accounts are deleted at settings > iCloud, messages accounts are deleted at settings > messages.